Why is it not possible to share the Spam folder?

We have an office account that would be managed by 5 people.
So I share the inbox and the outbox folder to them.
But I can't share the spam folder to them. But we have mails in this folder that should be checked.

But that's not a solution for us.
The Mails should be separated in different Accounts.

The user should have his own Mails in his Account. The office Mails should be in a separated Mailbox.
If the user moves to an other department it should be easy to remove all office Mails from his Account.

Also if a new User should get the Mails he should also see all old Mails.

To share the account is the only solution I found for this.
A solution for the Spam folder would be to move all Spam Mails in an extra Folder that can be shared.

But I don't understand why it is not possible to simply share the spam folder.

The spam folder is likely not sharable since it's integrated with SpamAssassin. If somebody drops a mail into that folder, its info is automatically added to the SpamAssassin database. I'm not sure how sharing that folder would affect SpamAssassin.

I understand that sharing one account is the process you want, but if it's not technically possible, then it's often easier to step back and reconsider the desired process given the limitations of the technology.

Continuing with my original suggestion, you could effectively accomplish that with user-level filters. Configure a filter for each user. If the TO field matches the distribution group address, the message is then filed into a folder under the user's Inbox with the name of the group. As an alternative to filtering the emails into a folder (where they might get overlooked), you could save a Zimbra search for all emails matching the distribution group address in the TO field.

I have to say, however, that I'm not sure what the reason for removing old department emails would be if a user changes departments, unless there may be sensitive department-specific email that should not be carried across departments. (But then, the user will have knowledge of that information moving between departments anyway.)

Anyway, doing a little more digging, it is possible to use a zmprov flag to effectively disable the spam filtering for a specific account: